An internal antenna is generally used in small-sized radio devices, such as mobile phones, in order to avoid a part protruding from the cover of the device. Internal antennas are usually planar antennas, because they have relatively good electric properties. A planar antenna comprises a radiating plane and a ground plane parallel with it. The planes are generally connected to each other by a short-circuit conductor because of the matching of the antenna. The structure is dimensioned so that it functions as a resonator at the operating frequency, which is a prerequisite for effective radiation. In modern mobile stations it is a normal requirement that the antenna must operate on two different frequency bands, in which case two resonators are also required. This requirement is met by dividing the radiating plane into two branches of different lengths by means of a non-conductive slot or area. Together with the ground plane and a medium, each branch forms a resonator, the natural frequency of which is arranged at one operating band of the radio device.
The radiating plane can be a separate metal sheet, in which case its slot is formed by cutting while the whole plane is cut from a larger sheet. Saving of material is achieved by manufacturing the radiating plane of thin metal foil. Then the radiating plane cut from the foil is, for example, glued onto the antenna's dielectric frame or onto the inner surface of the cover of a mobile station. The difficulty is to make the shape of the foil element remain exactly right during fastening. Even a relatively small change in the dimensions of especially the non-conductive area of the plane impairs the characteristics of the antenna significantly. The risk of changing the shape of the foil element is avoided if a dielectric plate coated by a metal foil is used for manufacturing the antenna. The desired radiator pattern is formed on the surface of the plate by etching away the surplus parts from the coating. The resulting antenna element is then fastened at a certain distance from the ground plane.
FIG. 1 shows a radiating antenna element 100 manufactured by the known method described above. It comprises a dielectric substrate 110 and a radiating plane 120, which is a conductor layer on the surface of the substrate. The radiating plane has an antenna feed point FP and a short-circuit point SP close to each other. From the latter, the radiating plane is directly connected to the ground plane when the antenna element is installed on place. The non-conductive area 130 starts from the same edge of the element beside which the feed point and the short-circuit point are, and divides the radiating plane into two conductor branches as seen from the short-circuit point SP. The first conductor branch 221 comprises the peripheral areas of the plane, forming a pattern resembling the letter C. The second, shorter conductor branch 222 comprises the inner area of the plane. The lower operating band of the antenna is based on the first conductor branch, and the upper operating band of the antenna is based on the second conductor branch. The antenna element has been cut to such a shape that it follows the inner space of the end part of the radio device in question. FIG. 1 shows the outline COV of the end part.
The non-conductive area 130 of the antenna element 100 has been formed by removing part of the conductive coating of the substrate by etching. The chemicals needed in the process cause a considerable cost in production. This drawback is emphasized if the area between the conductor branches is made relatively wide in order to increase the bandwidths of the antenna. Besides, the chemicals used are environmental poisons, the disposal of which causes additional costs. In principle, it could also be used laser for removing the conductor material in the known manner. However, laser suits well for making very narrow slots only. Removing a relatively wide conductor area would thus be impractical, i.e. expensive, and it would also impair the mechanical and electrical characteristics of the dielectric plate used as a substrate.